The use of semi-permeable membrane barriers in apparatus designed to accomplish liquid phase separation of a solvent from a solute is well known to those of skill in the art. Typical of such apparatus are those used in reverse osmosis, electro-dialysis, ultrafiltration, and similar processes. Unfortunately, in industrial applications of these processes, there is a tendency of the less soluble components in the solution to precipitate on the membrane barriers as the solution is concentrated. The precipitation of certain solutes, such as calcium sulfate and silica, is particularly deleterious since they form hard deposits on the membrane barriers which are extremely difficult, and sometimes impossible, to remove. The deposits which form on the membrane barriers in this manner greatly inhibit passage of solvents through the apparatus and, accordingly, decreases the overall efficiency of the separation unit.
Heretofore, numerous means have been proposed to reduce the problem of fouling in membrane-containing separation apparatus. For example, U.S. Pat. No. 3,552,574 discusses the problem of fouling of membranes in a reverse osmosis system and purports to solve the problem by providing a plurality of scouring elements, including free spheres made of glass, ceramics, stainless steel, noble metal, hard rubber, or plastics in the space adjacent the face of the semi-permeable membrane barrier. The spheres are agitated by the liquid which circulates across the membrane causing the spheres to oscillate within the space adjacent the membrane, thereby cleaning the membrane surface. Similarly, in U.S. Pat. No. 3,541,006, particulate glass and elastomeric beads are introduced into the upstream side of an ultrafiltration membrane unit to dislodge retained solute material from the membrane surface. Other proposed methods utilize sequesterants in the liquid medium to keep the membrane free from salt deposits. See, e.g., U.S. Pat. Nos. 3,394,068 and 4,046,686. Still other methods, such as that disclosed in U.S. Pat. No. 4,036,749, attempt to eliminate fouling by pre-treating the liquid medium prior to introduction into the membrane-containing unit, to remove ions which can form deposits in the unit.
While these methods for reducing fouling in membrane-containing separation units have been mildly successful in industrial applications, there remains a need for improved methods of maintaining such separation units clean and deposit-free.